The present invention relates to bicycle frames and, more particularly, to bicycle frames in which the components are joined together by means other than welding or brazing.
The frames for most lightweight touring or racing bicycles comprise a head tube which includes a sleeve for receiving the handlebar stem and fork stem, a bottom bracket which includes a sleeve for receiving the pedal crank assembly, and a seat bracket which includes an opening for receiving the seat post of the bicycle seat. These three components are joined together by single, cylindrical tubes which usually are joined to the components by welding or brazing.
The front wheel of the bicycle is attached to this frame by a front fork which includes a fork stem pivotally attached to the head tube. The rear wheel is attached to this frame by seat mast stays which extend from the seat bracket to the rear wheel, and a pair of chain stays which extend from the bottom bracket to the rear wheel. These two pairs of stays usually are attached to their respective brackets by welding or brazing.
In most bicycles of this design, the tubes, brackets and stays are made of steel, a material which is strong, inexpensive and relatively easy to weld or braze. However, there are other materials, such as aluminum alloys, which possess sufficient strength and rigidity and yet are lighter in weight than steel. Accordingly, to minimize the weight of the frame, it is desirable to construct a bicycle frame in which aluminum alloy tubes are substituted for some or all of the components. Due to their sizes, it is especially desirable to substitute aluminum for the steel in the tubes joining the brackets. Since the techniques for joining a steel component to an aluminum one by welding or brazing are impracticable for mass production, alternate means of constructing such frames have been utilized.
For example, a bicycle frame is disclosed in the Kleinebenne Gebrauchsmuster No. G 81 17 517.5 in which tubular members are attached to the head tube, bottom bracket and seat bracket by gluing. The head tube and each of the brackets include sockets into which are pressed, soldered or welded tubular "appendages" having tapered ends. The ends of the tubes are complementarily tapered and sized to slip over the appendages. The tubes are secured to the appendages by an anaerobic adhesive which hardens and solidifies to prevent relative movement of the parts without actually bonding to them, thereby forming a mechanical interlock. In a preferred embodiment, the two ends and appendages include circumferential grooves which together form annular passages for holding adhesive, in addition to the adhesive held in the gaps or pockets between the appendages and the tubes.
In an alternate embodiment of that device, the tubular appendages are cylindrical in shape and include roughened outer surfaces which create minute pockets for the adhesive. The appendages are sized to provide a slight press fit along substantially their entire length. With this embodiment, the frame components are held together initially by the slight press fit, then primarily by the hardened adhesive which fills the gaps between the tube ends and the appendages attached to the head tube and brackets.
While this type of frame construction allows dissimilar metals to be used and therefore enables frames to be constructed which are lighter in weight than correspondingly shaped frames which are made entirely of steel, there are disadvantages. For example, the annular passages on the conical appendages hold glue in an annular pattern, so that the adhesive solidifies in the form of rings. While such a pattern provides resistance to tensile forces acting on the connection, it is not as effective in resisting torsional forces.
In addition, in those frames in which the inserts include roughened surfaces that engage the connecting tubes with a press fit, it is dificult to ensure that the adhesive is evenly distributed about the periphery of the appendages, and often the act of inserting an appendage into a tube scrapes adhesive away from portions of the periphery, creating voids. Furthermore, it is often difficult to join a tube end to an appendage so that the end engages the socket wall to form a continuous connection. Difficulties in this respect result from normal variations in assembly and in the dimensions of the components. Failure to provide a proper connection results in the presence of gaps between the tube and the socket which detract from the aesthetic appearance of the frame, and in the presence of sharp edges which may injure the rider of the bicycle.
Accordingly, there is a need for a bicycle frame which is capable of being fabricated of components made of dissimilar metals such as steel and aluminum, without need of expensive and time-consuming brazing operations. There is also a need for a bicycle frame in which the components can be assembled rapidly and inexpensively with a uniformly strong connection between the tubes and appendages. Such a frame should also be capable of assembly so that there are no unsightly gaps between the frame components and the likelihood of sharp edges existing in the finished frame is small.